The present invention is directed generally to a variable phase shifter for millimeter wave signals, and more particularly to a variable phase shifter based on the slow-wave effect.
There is increased interest in millimeter wave signal propagation both for military and commercial applications. The millimeter wave region is of interest because there are certain windows in the atmosphere in the millimeter wave region that will propagate millimeter waves for great distances. Additionally, millimeter waves will propagate through chaff and rain. In view of this interest, it is highly desirable to have millimeter wave circuit components which are compatible with solid state processing methods, require reduced real estate area, have a decreased cost per device, and an increased yield The phase shifter is an integral component of millimeter wave receivers, as well as other key RF components such as antennas, amplifiers, transmit-receiver switches, local oscillators and filters Presently several elements like GaAs MESFETs, lumped capacitors and inductors, diodes, and different electrical length and impedance transmission lines are required to realize phase shifters with between three and five bit capability, with each bit being, for example, 22.5 degrees or 45 degrees. The number of lumped elements involved leads to high cost and manufacturing yield problems for the phase shifters Additionally, at millimeter wave frequencies, these lumped elements frequently will not yield the desired phase shift values.
One type of phase shifter suggested for use in the microwave frequency range is based on the slow wave electromagnetic effect. The principle of operation for a device based on the slow wave effect has been discussed at length in the papers by Hasegawa, H., Furukawa, M., and Yanai, H.; "Slow Wave Propagation Along A Microstrip Line On Si-SiO.sub.2 Systems", Proceedings of the IEEE, February 1971, pages 297-299; and Hasegawa, H , Furukawa, M., and Yanai, H.; "Properties Of Microstrip Line on Si-SiO.sub.2 System", IEEE Transactions, November 1971, Volume MTT-19, No. 11, pages 869-881. Slow wave propagation along a microstrip transmission line disposed over a semiconductor medium has been discussed at length in the papers by Hughes, G., and White, R.: "Microwave Properties Of Nonlinear MIS as Schottky Barrier Microstrip", IEEE Transactions, October 1975, Volume ED-22, pages 945-956; and Jager, D.: "Slow-Wave Propagation Along Variable Schottky-Contact Microstrip Line", IEEE Transactions, September 1976, Volume MTT-24, pages 566-573. The structure for such a phase shifter based on the slow wave effect comprises a Schottky microstrip line disposed over a semiconductor medium. A depletion layer is formed in the semiconductor medium directly below the microstrip line. The depth of this depletion layer depends on the reverse bias V applied between the microstrip line and the semiconductor medium. The slow wave effect occurs due to the different partitioning of the magnetic and electric field energy between the depletion layer and the semiconductor layer. In essence, the electric field is screened to some extent by the doped semiconductor region below the depletion layer. The magnetic field is also screened, but by a different amount. This different partitioning of the magnetic and electric energy in the two layers causes a slowing effect on electromagnetic energy propagating along the Schottky microstrip line. It has been found that simply by altering the thickness of one of the layers, the propagating phase velocity of the electromagnetic energy propagating along the Schottky microstrip line can be altered. Thus, variable phase velocity control can be achieved simply by controlling the thickness of the depletion region formed underneath the Schottky microstrip line.
Current slow-wave devices are designed for frequencies of two gigahertz or less. This frequency limitation is due to the fact that higher frequencies require a very thin semiconductor medium disposed below the Schottky microstrip line. The thick semiconductor mediums currently in use do not eliminate higher order mode effects (including those due to the microstrip line and the surface waves) as discussed in Krowne, C.; "Slow-wave Propagation in Two Types of Cylindrical Waveguides Loaded with a Semiconductor," IEEE Transactions, April 1985, Volume MTT-33, Pages 335-339. Accordingly, prior slow wave devices are unsuitable for use with millimeter wave signals.